The present invention relates generally to a multi-functional drive system, and more particularly to a photovoltaic (PV) integrated variable frequency drive system.
As the drive to reduce CO2 emissions gathers momentum, alternative energy or distributed sources are gaining importance. These distributed sources include wind, tidal energy, and solar energy. In particular, solar energy, where light energy from the sun is converted into electrical energy using photovoltaic (PV) cells, is becoming one of the most prominent alternative energy resources. In addition to reducing harmful emissions, these distributed sources contribute several ancillary services to the power industry, such as peak power shaving, backup power, and spinning reserve support.
PV-based systems are well known in the art. The most common structure of a grid-connected PV system is shown in FIG. 1. Most PV-based systems include a PV array, DC bus capacitors (not shown), an intermediate DC-DC converter, a DC-AC inverter, and an output filter. The PV arrays include solar cells which may be arranged in any series-parallel combination to obtain a desired DC voltage. The solar cells can also be integrated into buildings as a building-integrated PV (BIPV) system.
The DC-DC converter provides Maximum Peak Power Tracking (MPPT) and a regulated and/or boosted DC voltage for optimal operation of the DC-AC inverter. MPPT schemes are very important in a typical PV system because PV arrays are comprised of semiconductor solar cells whose I-V characteristics depend on the ambient temperature and the irradiance. Thus, the detection of the PV array voltage and current where the generated PV output power will be maximum is of paramount importance.
MPPT schemes automatically detect this optimal operating point and tracking is provided continuously through the operation of the PV system. Various techniques, such as hill-climbing/perturb and track approach, incremental conductance, and fractional open-circuit voltage/short-circuit current detection may be used in the MPPT schemes. These MPPT techniques may be integrated into the DC-DC converter or a separate PV reference cell may also be used.
While FIG. 1 shows a voltage-fed inverter topology, other inverter topologies such as current-fed inverter systems are also common and have a similar structure.
Although there has been a significant amount of research in the area of grid-tied PV systems, most PV-based applications have focused on single-phase integration to the grid. However, there has been some work associated with PV systems using variable frequency drives. Unfortunately, these applications are not multifunctional and use PV as the only source. The topology of one such PV system is shown in FIG. 2. Another application where the utility and PV array are interconnected to the load using a multi-input converter is shown in FIG. 3. This application requires that each source has its own converter.
Accordingly, there is a need for an efficient multi-functional drive system that can function as a grid-tied inverter while integrating a PV array.